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The act of defecating in the back tank of a toilet. The result is that whenever there is a flush, the bowl fills with tainted water.
Before I moved out, I top tanked the toilet in the master bedroom.
by Robert Smith August 06, 2004
The act of shitting in the cistern of an enemy's toilet to ruin their bathroom for a thousand flushes.
Shortly before they sealed off Hitler's bunker beneath Berlin, Larry Silverstein toptanked every toilet available and made good on his escape.
by Kenny Evil June 30, 2010
Taking a dump in the systern of a toilet. Holding youself up by you hands on the wall and dropping in a bombay bad boy. The only way that can be fixed is if a glass-monkey mops the whole stinky area out.

Particularly skilled people can piss at the same time and even sometimes get a few drops in the bowl.
Norm gave Gilhooley a top tank for his birthday. He thought and animal (a trained dog) had done it.
He was wrong.
by Matt Regsmith April 30, 2003